Effects of boundary conditions on vortex breakdown in compressible swirling jet flow simulations

• High sensitivity of vortex breakdown to inflow boundary conditions, less to outflow boundary conditions.
• Any artificial disturbances introduced by boundary conditions should be avoided.
• Non-reflecting characteristic boundary conditions (NSCBC) together with sponge-layers are the best choice.

We investigate the sensitivity of numerical simulation results for swirling jet flows undergoing vortex breakdown to inflow and outflow boundary conditions. The compressible regime at Mach number Ma=0.6Ma=0.6 and Reynolds number Re=5000Re=5000 is considered. The swirl velocity is approximately of the same magnitude as the streamwise centreline velocity at inflow. We perform Large-Eddy Simulations using high-order discretization schemes in space and time. A rotating nozzle with isothermal wall is included in the computational domain. Six different combinations of inflow and outflow boundary conditions are investigated. These use a Dirichlet condition at the inflow supplemented with a sponge layer imposing up to five variables and a sponge layer at the outflow acting on several combinations of variables, applied together with non-reflecting boundary conditions. The advantages and drawbacks of each setup are investigated. The qualitative features of the swirling jet undergoing vortex breakdown are robust to changes in the inflow and outflow boundary conditions, i.e., conical shear-layers, a recirculation bubble, the existence of a single-helix type instability, and the occurrence of a dominant frequency, are all captured by combinations of the boundary conditions investigated. However, significant quantitative differences are observed depending on the conditions set at inflow and outflow. In particular, the locations of the stagnation points and the spreading angle of the swirling jet are strongly influenced. The size and shape of the recirculation bubble change as well, as does the intensity of the recirculation flow and of the counter-rotating motion observed at the jet centreline. The dominant frequency in the breakdown region also depends on the setup. As a result of this study, we recommend setting the three velocity components, density, and pressure at the inflow and outflow using sponge layers supplementing non-reflecting boundary conditions as the most suitable choice.